Loop execution flow - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a loop in C programming?

A loop repeats a block of code multiple times until a condition is no longer true.

beginner

Describe the flow of a for loop in C.

A for loop starts by initializing a variable, then checks a condition before each iteration, executes the loop body if true, and updates the variable after each iteration.

beginner

What happens if the loop condition is false at the start of a while loop?

The loop body does not execute even once, and the program continues after the loop.

intermediate

How does a do-while loop differ in execution flow from a while loop?

A do-while loop executes the loop body first, then checks the condition, so it always runs at least once.

intermediate

What is the role of the break statement inside a loop?

The break statement immediately stops the loop and transfers control to the code after the loop.

What part of a for loop runs only once at the beginning?

A Initialization

B Condition check

C Loop body

D Increment/Update

The initialization runs once before the loop starts.

When does the condition in a while loop get checked?

A Only once at the start

B After each iteration

C Before each iteration

D Never

The while loop checks the condition before running the loop body each time.

Which loop guarantees the body runs at least once?

A <code>while</code> loop

B <code>do-while</code> loop

C <code>for</code> loop

D None of these

The do-while loop runs the body first, then checks the condition.

What happens if the loop condition is false at the start of a for loop?

A Loop body runs once

B Program crashes

C Loop runs infinitely

D Loop body does not run

If the condition is false initially, the loop body does not execute.

What does the break statement do inside a loop?

A Ends the loop immediately

B Pauses the loop

C Restarts the loop

D Skips current iteration

The break statement stops the loop right away.

Explain the execution flow of a for loop in C.

Think about what happens first, then what repeats.

Describe the difference in execution flow between a while loop and a do-while loop.

One checks condition before running, the other after.

Practice

1. Which loop in C always executes its body at least once, regardless of the condition?

easy

A. do-while loop

B. for loop

C. while loop

D. if statement

Solution

1. Step 1: Understand loop types

   The do-while loop checks its condition after running the loop body once.

2. Step 2: Compare with other loops

   The for and while loops check the condition before running the body, so they may not run at all.

3. Final Answer:

   do-while loop -> Option A

4. Quick Check:

   do-while runs body once before condition check [OK]

Hint: do-while runs body first, then checks condition [OK]

Common Mistakes:

• Thinking for or while loops always run once
• Confusing if statement with loops
• Assuming do-while checks condition first

2. Which of the following is the correct syntax for a for loop in C that counts from 0 to 4?

easy

A. for (i < 5; i = 0; i++) {}

B. for i = 0 to 4 {}

C. for (int i = 0; i < 5; i++) {}

D. for (int i = 0; i <= 5; i++) {}

Solution

1. Step 1: Check for loop syntax

   The correct syntax is for (initialization; condition; increment). for (int i = 0; i < 5; i++) {} matches this.

2. Step 2: Validate loop range

   for (int i = 0; i < 5; i++) {} counts i from 0 up to but not including 5, so 0 to 4.

3. Final Answer:

   for (int i = 0; i < 5; i++) {} -> Option C

4. Quick Check:

   Standard for loop syntax with i from 0 to 4 [OK]

Hint: for loops use (init; condition; increment) syntax [OK]

Common Mistakes:

• Using 'for i = 0 to 4' which is not C syntax
• Swapping condition and initialization
• Using <= 5 to count 0 to 4 (includes 5)

3. What is the output of this C code?

int i = 0;
while (i < 3) {
    printf("%d ", i);
    i++;
}

medium

A. 0 1 2

B. 1 2 3

C. 0 1 2 3

D. 3 2 1 0

Solution

1. Step 1: Analyze loop condition and variable

   Variable i starts at 0 and increments by 1 each loop while i < 3.

2. Step 2: Trace output values

   Loop runs for i = 0, 1, 2 printing each value followed by a space.

3. Final Answer:

   0 1 2 -> Option A

4. Quick Check:

   while loop prints 0,1,2 then stops [OK]

Hint: Check loop condition carefully to count printed values [OK]

Common Mistakes:

• Including 3 in output by mistake
• Starting count from 1 instead of 0
• Printing after increment instead of before

4. Identify the error in this loop code:

int i = 0;
while (i < 5) {
    printf("%d ", i);
}

medium

A. Incorrect printf format specifier

B. Missing increment of i inside the loop

C. Condition should be i <= 5

D. Variable i should be declared inside the loop

Solution

1. Step 1: Check loop body for changes to i

   The variable i is never incremented inside the loop, so i stays 0 forever.

2. Step 2: Understand loop consequence

   Without increment, the condition i < 5 is always true, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the loop -> Option B

4. Quick Check:

   Infinite loop due to no i++ [OK]

Hint: Always update loop variable to avoid infinite loops [OK]

Common Mistakes:

• Thinking condition is wrong instead of missing increment
• Assuming printf format is incorrect
• Declaring i inside loop is optional, not error

5. Consider this nested loop:

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= i; j++) {
        printf("%d", j);
    }
    printf("\n");
}

What is the output?

hard

A. 1
22
333

B. 1
2
3

C. 123
123
123

D. 1
12
123

Solution

1. Step 1: Understand outer loop iterations

   The outer loop runs i from 1 to 3, inclusive, so 3 times.

2. Step 2: Trace inner loop for each i

   For each i, inner loop runs j from 1 to i, printing j values without spaces.

3. Step 3: Combine outputs

   Lines printed are:
   i=1: "1"
   i=2: "12"
   i=3: "123"
   Each followed by newline.

4. Final Answer:

   1 12 123 -> Option D

5. Quick Check:

   Nested loops print increasing sequences per line [OK]

Hint: Inner loop runs up to outer loop counter each time [OK]

Common Mistakes:

• Confusing inner loop range with outer loop
• Printing j values incorrectly
• Missing newline after each inner loop